Bump actions/checkout from v1 to v2

[xt0rted]

There were changes in how the checkout is done between v1 and v2. This shouldn't effect the ci workflow but I'm not sure about the release workflow.

https://github.com/actions/checkout/releases/tag/v2.0.0

Changelog

• Improved fetch performance
  • The default behavior now fetches only the commit being checked-out
• Script authenticated git commands
  • Persists the input token in the local git config
  • Enables your scripts to run authenticated git commands
  • Post-job cleanup removes the token
  • Opt out by setting the input persist-credentials: false
• Creates a local branch
  • No longer detached HEAD when checking out a branch
  • A local branch is created with the corresponding upstream branch set
• Improved layout
  • The input path is always relative to $GITHUB_WORKSPACE
  • Aligns better with container actions, where $GITHUB_WORKSPACE gets mapped in
• Fallback to REST API download
  • When Git 2.18 or higher is not in the PATH, the REST API will be used to download the files
• Removed input submodules

[stefanbuck]

I can't see any potential issue so let's get this merged and we will see soon wether this is working or not.